MEMS microphone chips can currently be divided into piezo-resistive type, piezo-electric type, and capacitive type, among which the capacitive MEMS microphone has the widest range of applications. The capacitive MEMS microphone possesses the following advantages: small size, high sensitivity, good frequency characteristic, low noise, and wide working temperature range.
The capacitive MEMS microphone is generally consisted of a back plate and a diaphragm. The diaphragm has a certain flexibility and can be vibrated by virtue of air. The back plate has certain rigidity and fully defines a plurality of holes which are so called acoustic holes. Air can pass through the plurality of acoustic holes to enable the diaphragm to vibrate, while the back plate will not vibrate along with the diaphragm. The back plate and the diaphragm cooperatively form a plate capacitor, the sound drives the flexible diaphragm to vibrate by the air, thereby changing the capacitance value of the plate capacitor. The change of the capacitance value generates a detectable electrical signal for the peripheral circuit, thereby enabling the conversion from the sound signal to the electrical signal. Sensitivity is an important indicator to evaluate the capacitive MEMS microphone. The sensitivity of the silicon-based microphone is determined by the size of the diaphragm, the extent of deformation and the shape of the deformation. At present, if other design structures remain unchanged, the sensitivity of the capacitive MEMS microphone will inevitably be reduced if the chip area is to be reduced.